Methods presently exist for removing contaminants from underground areas. Excavation, treatment and reburial of the contaminated soil is one commonly applied method. However, the applicability of physical excavation of contaminated soils to most contaminated sites is limited. The process of excavation and treatment is very expensive and highly disruptive to any on-going plant operations. Furthermore, excavation is limited by practical considerations to relatively shallow depths.
In-situ liquid leaching of water soluble contaminants is another, less frequently applied method. Russomano (U.S. Pat. No. 4,611,950) and Forte et al. (U.S. Pat. No. 4,167,973) describe techniques for removal of contaminants from contaminated soil by liquid leaching. These approaches are generally limited to water soluble contaminants with low soil adsorption. Furthermore, the rate of water flow through the contaminated soil is limited to that obtainable by gravity drainage through the unsaturated zone, which can be very slow, limiting the effectiveness of the leaching process. Finally, the leaching process transports contaminants from the unsaturated zone to the water table, contaminating the saturated zone, which must then also be remediated.
In-situ soil vapor extraction, whereby soil gases containing volatile contaminants are extracted from the subsurface soils, has become an increasingly popular method over the past few years. The concept of venting soil gases from the unsaturated zone was generally first applied to control of landfill gases. Croskell (U.S. Pat. No. 4,518,399) provides a typical landfill gas recovery system design. Zison (U.S. Pat. No. 4,469,176) provides an improved landfill gas recovery process comprising controlling the pressure in the gas collection zone. Later, practitioners applied soil gas venting as a mechanism for remediation of contaminated subsurface soils. Knopik (U.S. Pat. No. 4,183,407) describes a vertical conduit with radial extensions that is used to exhaust underground contaminant vapors. The major shortcomings of this design are that it requires potentially disruptive excavation and that it is limited to shallow depths where excavation and emplacement of the radial conduits are feasible. These shortcomings were addressed by Visser et al. (U.S. Pat. No. 4,593,760, 4,660,639 and Re 33,102) and Payne (U.S. Pat. No. 4,730,672), who describe cased air wells containing conduit perforated in a specified interval above the water table from which soil vapors may be extracted or into which clean air may be injected. These designs can operate at arbitrary depths and are less disruptive, requiring no extensive areal excavation.
The use of soil vapor extraction to enhance biodegradation was disclosed by Ely and Heffner (U.S. Pat. No. 4,765,902) who describe the use of air wells to draw oxygen into a contaminated zone to stimulate microbial biodegradation of hydrocarbons. These wells are similar in construction to those disclosed in the patents by Visser et al. and by Payne.
The designs based on air wells containing perforated conduit (well casing) are deficient in several respects: First, owing to the air extraction's lowering of air pressure at the well, and therefore, a lowering of air temperature below that of the soil, some of the near-saturated water vapor drawn from the soil will condense onto the casing surfaces or into droplets in the air stream within the well. The low pressure can also cause liquid water in the soil to migrate and accumulate in the vicinity of the well; a portion of this water can then be drawn into the air well. As the air stream within the well casing moves to the land surface it entrains a portion of the water that has entered or condensed in the well. This water is brought to the surface and must be separated from the air stream. The separated water, which contains contaminants, must then be treated and disposed of, incurring additional disposal methods and costs. Secondly, over time, water carried to the well but not to the surface can accumulate in sufficient amount to cause downward infiltration of contaminated water and spread of contamination to soils or groundwater below the well. Thirdly, when an air well is operated in close proximity to the water table, the low pressure in the well can cause significant upwelling of the water table directly below the well, interfering with the well efficiency and increasing the quantity of water entrained in the extracted air stream.
Accordingly, there is the need for a process that will remove contaminant compounds from the soil by drawing air through the vadose zone of the soil and to the land surface, but will minimize the transport of contaminated water to the surface and/or to soils or groundwater beneath the contaminated zone and also will minimize upwelling of the water table where it is in close proximity to the contaminated vadose zone. The need has now been satisfied by the invention that is described below.